1. Field of the Invention
The present invention relates to an optical data recording medium for optically recording data therein and for optically reproducing the data recorded therein. More specifically, the present invention relates to an optical data recording medium capable of recording information therein with high recording density and capable of reproducing the information recorded therein with high reproducibility.
2. Description of Related Art
One type of a conventional optical data recording medium is shown in FIG. 1. The optical data recording medium 80 includes a transparent substrate 82 in which a tracking guide groove 81 is formed in concentrical or spiral fashion, a first protective layer 83 formed over the substrate 82, an optical data recording layer 84 formed over the first protective layer, and a second protective layer 85 formed over the data recording layer 84. The substrate 82 is made of acrylic resin, polycarbonate resin, glass, or the like. The first protective layer 83 is formed of oxide material such as SiO.sub.2, nitride material such as AlN, or the like. The data recording layer 84 is made of magnetooptic material such as TbFeCo, GdTbFe and TbCo, phase-change material such as GeSbTe, pit formable material such as metal and dye, or the like. The second protective layer 84 is formed of oxide material such as SiO.sub.2, nitride material such as AlN, resin, or the like.
In the data recording medium 80 having the above-described structure, as illustrated in FIG. 2, the tracking guide groove 81 is formed so that a groove width W.sub.g, a groove-to-groove distance W.sub.L, and a track pitch P may satisfy the following equation P=W.sub.G +W.sub.L and so that the track pitch P may be set to be approximately equal to a spot diameter dS of a laser beam spot to be irradiated on the data recording medium 80. In addition, as shown in FIGS. 4 and 5, preformat pits 87 are formed on land portions 86 of the substrate 82.
The substrate 82 thus formed with the tracking guide groove 81 and the preformat pits 87 is produced through the following manner.
First, a photoresist with its thickness having a value equal to or lower than 0.1 .mu.m, for example, is formed over a glass plate, through a spin coating process. The photoresist is irradiated with laser beam in such a manner that a laser beam spot formed on the photoresist is moved relative to the photoresist concentrically or spirally with a fixed pitch. The laser beam irradiation is performed continuously and intermittently. The continuously irradiated laser beam will form a groove extending concentrically or spirally and the intermittently irradiated laser beam will form pits which are arranged concentrically or spirally. Then, the photoresist is subjected to a development process, so that a mask member consisting of the glass substrate and the photoresist formed with the groove and the pits is obtained.
Then, an electro-conductive film is provided over the mask member before being subjected to an electro-forming process. Thus, a nickel stamper is produced. The nickel stamper is then subjected to resin molding operation such as a photopolymerization process or an injection molding process. Accordingly, resin is molded into the substrate 82 onto which are transferred the groove and the pits. Thus, the substrate 82 formed with the tracking guide groove 81 and the preformat pits 87 is obtained.
In order to record data into the data recording medium 80, on the other hand, as shown in FIG. 1, laser beam is irradiated on the data recording layer 84 through the substrate 82 to locally heat the data recording layer. As a result, the heated portion of the data recording layer undergoes inversion of magnetization, phase change, or formation of pit. Thus, a data recording bit is formed in the data recording layer. In order to reproduce the data thus recorded in the data recording layer 84, a laser beam is also irradiated on the data recording layer 84, and the laser beam reflected from the data recording layer is detected. More specifically, change in rotation angle of a polarization plane of the reflected laser beam or change in intensity of the reflected laser beam is detected.
In order to increase a data recording density of the above-described data recording medium 80, it is necessary to narrow the track pitch P. In the case where the track pitch P becomes smaller than the spot diameter dS of the laser beam spot 91, however, the following problem will occur. As shown in FIG. 3, in the case where the laser beam spot 91 traces a particular track 92 in order to reproduce data recorded in the track, the laser beam spot 91 fails to be irradiated only on the track 92 but is irradiated not only on the track 92 but also a neighboring track 93. The laser beam spot is therefore erroneously irradiated onto a part of a data recording bit 94 which is formed in the neighboring track 93. Accordingly, the laser beam spot tracing the particular track 92 picks up not only desired information recorded on the particular track 92 but also unwanted information on the neighboring track 93. Thus, cross-talk is occurred.
In addition, the conventional data recording medium 80 has a problem that the tracking guide groove 81 and the preformat pits 87 formed on the substrate 82 through the injection molding process are liable to have undesired shapes. In other words, a yield rate of the data recording medium 80 has a small value. This tendency would be particularly acknowledged if the groove width W.sub.G is decreased to enhance the data recording density of the data recording medium.
FIG. 6 shows one example of the above-described conventional optical data recording medium 80 which applies magnetooptic material as the data recording layer 84. The conventional magnetooptic data recording medium 170 includes a transparent substrate 172 made of polycarbonate resin or the like which is formed with a tracking guide groove 171, an interference layer 174 made of SiAlON, AlN or the like which is formed over the substrate 172, a magnetooptic data recording layer 176 made of GdTbFe, TbFeCo or the like which is formed over the interference layer 174, a protective layer 178 made of SiAlON, AlN or the like which is formed over the data recording layer 176, and a reflective layer 180 made of Al or the like which is formed over the protective layer 178. In order to record data in the data recording medium 170, laser beam is irradiated on the data recording layer 176 to locally heat it to a temperature equal to or higher than its Curie temperature or its Compensation temperature. Simultaneously, magnetic field is applied to the heated portion of the data recording layer 176, to thereby invert a direction of magnetization exhibited in the heated portion.
In order to reproduce the data thus recorded in the data recording layer 176, a linearly polarized laser beam is irradiated on the data recording layer 176 through the substrate 172. A polarization plane of the linearly polarized laser beam is rotated by a Kerr rotation angle at the time when the laser beam is reflected at the data recording layer 176. A direction in which the polarization plane is rotated at the Kerr rotation angle depends on the direction of the magnetization occurred in the data recording layer 176. Accordingly, in order to reproduce the data recorded in the data recording layer, the rotating direction of the polarization plane of the laser beam is detected.
In the data reproducing operation, the laser beam irradiated on the data recording medium from its substrate side is partly reflected at an interface defined between the substrate 172 and the interference layer 174, at an interface defined between the interference layer 174 and the data recording layer 176, at an interface defined between the data recording layer 176 and the protective layer 178, and at an interface between the protective layer 178 and the reflective layer 180. In other words, the laser beam undergoes multiple reflection both in the interference layer 174 and the protective layer 178. As a result, a plurality of reflective laser beams are generated at the respective interfaces and travel back to the substrate 172. The plural reflective laser beams meet one another and interfere with one another, to thereby constitute a single reflective laser beam to be outputted from the data recording medium 170. In the conventional data recording medium 170, thickness of the interference layer 174 is selected to approximately .lambda./(4n.sub.2) where n.sub.2 represents an index of refraction of the interference layer and .lambda. represents a wavelength of the laser beam in vacuum space so that the reflectivity of the interference layer 174 may be minimized. In addition, the thickness of the data recording layer 176 is selected to such a value as minimizes the reflectivity of the data recording layer. Since the data recording medium 170 has such a structure, the interference occurred among the reflective laser beams generated at the respective interfaces undergoes destructive interference, to thereby largely increase an apparent Kerr rotation angle of the laser beam outputted from the data recording medium. (This phenomenon is called as a "Kerr effect enhancement", hereinafter.) As a result, carrier-to-noise ratio (C/N ratio) of the data recording medium is largely enhanced.
The conventional data recording medium 170, however, has the following disadvantage. The thickness of the interference layer and the data recording layer are selected to such values as minimize the reflectivities of the interference layer and the data recording layer, as described above. Thus, the interference layer and the data recording layer have reflectivities of low values, at their entire areas. Thus, the conventional data recording medium 170 has a low value of reflectivity, even at its tracking guide groove 171. It is noted, however, that intensity of reflective laser beam reflected from the tracking guide groove 171 is detected and a tracking error signal is produced for tracking servo operation. Thus, the conventional data recording medium 170 having the low value of reflectivity at its tracking guide groove may not obtain a tracking error signal having an amount proper to attain a stable tracking servo operation. The data recording medium therefore may not attain a stable recording and reproducing operations.